1. Field of the Invention
The present invention relates to a centrifugal clutch for use mainly in a compact combustion engine used to drive a work machine such as, for example, a brush cutter.
2. Description of Related Art
In the compact combustion engine of a type utilizing a centrifugal clutch, the clutch starts engaging when the combustion engine attains the rotational speed of, for example, 3,800 rpm and is completely engaged when the combustion engine attains the rotational speed of about 7,000 rpm. It is, however, often experienced that during a semi-clutch period subsequent to the engagement of the clutch to the complete engagement of the same, a torsional vibration tends to be generated in, for example, a drive shaft and/or a clutch drum used to connect between the combustion engine and a load such as, for example, cutter blade. Such a torsional vibration occurs in the following manner by way of example.
When the clutch starts engaging incidental to an increase of the rotational speed of the combustion engine, the combustion engine is loaded with the cutter blade with the rotational speed of the combustion engine reduced consequently and, therefore, the clutch is likely to disengage. Even under this condition, the cutter blade continues its rotation by the effect of an inertia force. However, once the clutch is disengaged, the rotation of the cutter blade is lowered and the rotational speed of the combustion engine increases again, causing the clutch to be engaged again. At this time, the combustion engine is loaded with the cutter blade, accompanied by reduction in rotational speed of the combustion engine, which subsequently results in re-disengagement of the clutch. If the brush cutter is used during the semi-clutch period (with the rotational speed in a range of 3,800 to 7,000 rpm) referred to above, the clutch undergoes a cyclic repetition of engagement and disengagement in the manner described above and, due to a consequent change in rotational speed of the combustion engine and/or a change in torque, which are caused by the above discussed cyclic repetition, the torsional vibration occurs in the drive shaft and this leads to a body waggling of a main pipe used to support the drive shaft.
In an attempt to suppress the torsional vibration referred to above, the JP Laid-open Patent Publication No. 2009-228756 suggests the use of a torsional spring between the combustion engine and the clutch. In this combustion engine, a clutch shoe is supported through a torsional coil spring by a coupling shaft that is threadingly connected with an end portion of the output shaft of the combustion engine. The coupling shaft also functions as a connecting member for fixing a flywheel (rotor) to the output shaft. According to the above mentioned patent document, the torsional motion is absorbed by the torsional coil spring to thereby suppress the torsional vibration of the drive shaft.
It has, however, been found that since according to the above mentioned patent document the coupling shaft is threadingly connected with the end portion of the output shaft of the combustion engine and the clutch shoe is then supported by this coupling shaft, a spring force of the torsional spring acts on the coupling shaft as a rotating force while the coupling shaft receives the total weight of the clutch shoe. Accordingly, it may occur that the coupling shaft threadingly connected with the output shaft of the combustion engine may be loosened. Considering that the coupling shaft has a function as the connecting member for fixing the flywheel to the output shaft of the combustion engine, loosening of the coupling shaft in the manner described above results in a rattling motion taking place in the threaded connection between the coupling shaft and the output shaft of the combustion engine, accompanied by generation of obnoxious sounds brought about by vibrations of the flywheel. Also, the clutch may become unstable to such an extent as to amplify the vibrations.